It is on the principle of _stratification_ that most archaeological work must rest. The word means the characteristic of being in layers or strata. The usefulness of stratigraphic studies lies in the fact that in any undisturbed deposit the lowest layer or stratum will be the oldest since it was laid down first. This may be shown graphically by piling books on a table, one by one. The book at the bottom of the pile must inevitably have been put in place before the ones on top. The same principle is applied to ancient cultures. If the remains of one people are found underlying those of another, those on the bottom are older. Rarely are the remains of many cultures found lying directly over each other in a complete series but through correlation between sites the sequence may be established. For example, if in one place we find remains of Culture A underlying those of Culture B and in another place find material from Culture B underlying that of Culture C we may postulate that C is more recent than A even though the two are not found together. In still another place C may be found to underlie D and eventually a long sequence will be established, although it may not be present in its entirety in any one place. Objects acquired through trade are also useful in dating sites. For example, if we know the relative or absolute date at which a certain type of pottery was being made at one site, then find pieces of this ware at a site which we are trying to date we may assume at least some degree of contemporaneity. Stratigraphic studies, of course, do not provide us with absolute dates and for those we must turn to dendrochronology or tree-ring dating.[23] [121] The story of the development of this method is a strange one. It is a tale of an astronomer and archaeologists, of buried treasure that was only wood, of sun spots, and of purple chiffon velvet. Most important of all was the astronomer, for it was in his keen mind that the idea was born that was to lead to one of the most exciting scientific discoveries of our time.